The behavior of individual Na channels in the apical membrane of the rat cortical collecting tubule (CCT) was studied at different concentrations of the permeant ions Na and Li. Tubules were opened to expose their luminal surfaces and bathed in K-gluconate medium to minimize tubule-to-tubule variation in cell membrane potential and intracellular Na concentration. The patch-clamp technique was used to resolve currents through individual channels. The patch-clamp pipette was filled with solutions containing variable concentrations of either NaCl or LiCl. In one series of experiments, the concentrations were changed without substitutions. In another series, the ionic strength and Cl concentration were maintained constant by partial substitution of Li with N-methyl-D-glucamine (NMDG). In cell-attached patches, both the single-channel conductance (g) and the single-channel current (i) saturated as functions of the Na or Li activity in the pipette. Without NMDG, the saturation of i was well described by Michaelis-Menten kinetics with an apparent Km of approximately 20 mM activity for Na and approximately 50 mM activity for Li. Km was independent of voltage for both ions. With substitution for Li by NMDG, the apparent Km value for Li transport through the channels increased. The values of the probability of a channel's being open (Po) varied from patch to patch, but no effect of pipette ion activity on Po could be demonstrated. A weak dependence of Po on membrane voltage was observed, with hyperpolarization increasing Po by an average of 2.3%/mV.